The operation of subtraction is the same as "adding the opposite".
In other words, subtracting a positive number from something is the same as adding its negative.
EGs:
10 - 4 is the same as 10 + (-4)
3 - 1 is the same as 3 + (-1)
Likewise, subtracting a negative number from something is the same as adding its positive.
EGs:
10 - (-4) is the same as 10 + 4
3 - (-1) is the same as 3 + 1
Therefore, to rewrite subtraction as addition, we must change the sign of the number.
If you're still unsure how this works, please let us know.
Cheers ~ Mark :cool: